Version 0.5.1 released

Change persistence behaviour. The database is now stored in memory even if
persistence is enabled. It is written to disk when mosquitto exits and also
at periodic intervals as defined by the new autosave_interval option. This
makes persistence more suitable when being used on devices with a limited
number of writes, such as flash.

Default sqlite3-pcre path on Linux is now /usr/lib/sqlite3/pcre.so to match
new sqlite3-pcre packages.

The less interesting/bug fixes:

No longer store QoS=0 messages for disconnected clients that do not have
clean start set.